WASHINGTON (Reuters) - U.S. officials on Thursday defended the deadly shooting of a threatening air passenger as necessary to protect the flying public and the White House said an investigation will determine whether there were lessons to be learned from the incident.

The wife of Rigoberto Alpizar, the man who was shot and killed by federal air marshals at Miami International Airport on Wednesday, told the marshals her husband suffered from bipolar depression.

The two federal air marshals involved in the shooting of an American Airlines passenger were put on paid leave while the incident at Miami International Airport was investigated, said air marshal spokesman Dave Adams.

``In any law enforcement shooting, it's not just with the federal air marshals, they have to conduct an internal investigation,'' Adams said. ``At this time the (marshals) are on paid administrative leave pending the outcome of this investigation.''

The incident at the airport was the first time a passenger had been shot by air marshals since the enforcement program was beefed up after the September 11, 2001, hijacked airline attacks.

``This incident demonstrates the critical role that air marshals play in aviation security today,'' Homeland Security spokesman Russ Knocke said.

Officials said the passenger, identified as Rigoberto Alpizar, 44, indicated he was carrying a bomb in his bag as he boarded the plane for a flight to Orlando.

Adams said the two air marshals followed ``textbook training'' when they shot Alpizar after he ran off the airplane and ignored demands to stop and put down his bag.

Witnesses said Alpizar appeared to be mentally ill. Law enforcement officials said later there was no sign of a bomb.

Alpizar's family issued a statement on Thursday calling him ''loving, gentle and caring'' and said he was born in Costa Rica and was a naturalized U.S. citizen. One of the man's brothers, Rolando Alpizar, said his family in Costa Rica would demand an explanation from the U.S. government.

Miami-Dade police department issued a statement on Thursday saying Alpizar's wife had told them her husband had been diagnosed with bipolar disease. Rolando Alpizar said he had no knowledge that his brother suffered from a mental illness.

Alpizar was returning from Ecuador, where he had served as a translator for a group of dentists and ophthalmologists offering their services to poor communities, his brother said.

'PROTOCOLS' FOLLOWED

White House spokesman Scott McClellan said the marshals appeared to have acted in a way that was consistent with their training and said the FBI, Department of Homeland Security and police in Miami were investigating.

``Any time there's an investigation like this you learn lessons and then you apply those to future training and protocols,'' he said. ``But it appears that the protocols that were in place were followed in this situation.''

Adams said the two marshals, who have been with the service since 2002, followed agency policy.

``We don't have a shoot to kill policy, we have a shoot to take care of the threat, and that's what they did,'' he said.

Aviation security expert Charles Slepian said the incident raises questions of how to ensure airplane safety.

``The reason we need federal air marshals is because we know we are not being successful at keeping weapons or explosives off planes,'' he said.

``From an aviation security point of view we need to look at how we control what goes on in the cabin of an airplane.''

The deadly shooting evoked memories of London police in July shooting dead a Brazilian man at an underground station because they said they had thought he was a suicide bomber. The unarmed man was apparently running to catch a train.

It also raised questions over whether an alternative method -- like stun guns -- should be used in anti-terrorism efforts.

After September 11, U.S. airlines considered nonlethal weapons for flight crews but the government beefed up the air marshal program and began arming some pilots with firearms.

Tom Smith, president of stun-gun maker Taser International Inc., said Tasers should be put on planes in addition to firearms as a way to subdue passengers.

Convicted ``shoe-bomber'' Richard Reid was subdued by crew and passengers aboard a U.S.-bound transatlantic flight after attempting to detonate a bomb in his shoes. In 2002, a man who tried to break into the cockpit of a United flight was fended off by passengers and a co-pilot wielding a fire ax.